Various situations arise wherein one seeks to pump a liquid or gas from a low pressure source, such as a flow line, and output the fluid to a downstream container, such as a sample bottle. The sample bottle may be periodically sent to a laboratory for analysis to determine the BTU content of the sampled fluid, and thereby determine the BTU content of the gas flowing in the low pressure line. In many situations, a separate power source, such as an electric pump or pressurized hydraulic line, is readily available for driving the pump. In other situations typified by remote applications, a separate power source is not readily available or is not cost-effective for driving or powering the pump. In these latter situations, the low pressure source can be used to drive the pump, thereby avoiding the expenses associated with a separate power supply. Sampling pumps have typically been powered by an operator unit which receives low pressure from the downstream source, and which uses the low pressure as the driving force. Examples of sampling pumps which may utilize the downstream pressurized line as the driving force for the pump operator are disclosed in U.S. Pat. Nos. 4,928,536 and 5,032,063.
Many of the advantages of a sampling pump powered by an upstream fluid pressure frequently are not realized if the upstream fluid is contaminated with particles, such as rust, scale, or other particulate. The fluid ideally is filtered before entering the pump to reduce maintenance costs, and this filter ideally is closely adjacent the pump inlet port check valve. If the fluid line to the pump operator unit is not also filtered, however, service costs for the operator unit can become excessive. Service personnel periodically change the pump inlet filter, and may not inspect or change the operator unit filter.
In some applications, the pump inlet filter may be cleaned by the lower pressure fluid itself, as disclosed in U.S. Pat. No. 5,074,154. Much of the fluid flowing to a sampling pump may thus pass by, rather than through, the pump inlet filter in a "hot loop", thereby continually cleaning the pump inlet filter. In other applications, this hot loop technique is not feasible, and substantially all fluid flow to the pump is input and discharged from the pump.
In many applications, a check valve may be provided within the flow line downstream from the sampling pump, and in these cases an adjustable check valve may be utilized to control the downstream fluid pressure. An adjustable check valve offers a significant advantage in that the combination of (1) downstream fluid pressure desired as a result of the pumping operation, and (2) the efficiency of the pumping operations, may be maximized by using this check valve to adjust the downstream pressure. When used for gas applications, however, the pump discharge check valve ideally is closely adjacent the pump plunger, thereby minimizing the "dead areas" within the pumping system and improving pump efficiency, as disclosed in U.S. Pat. No. 5,074,154.
Various techniques have been employed to adjust the pumping capacity, i.e., the volume of each pump stroke. The '154 patent referenced above, for example, discloses a micrometer for adjusting the stroke of the operator piston, and thus the pumping plunger and thus the volume of the pump stroke. Also, it is well known to provide a purge system for a pump, so that fluid from the pump inlet may periodically effectively bypass the pump and go directly to the pump outlet, as disclosed in U.S. Pat. Nos. 4,440,032, and 5,074,154. Improved techniques are required, however, to provide a downstream fluid pressure pump with a lower cost pump stroke adjustment, and to reduce the expense associated with providing an improved pump with a purge system.
The disadvantages of the prior art are overcome by the present invention, and a novel pump is hereinafter disclosed to satisfy the need for a low cost and reliable pump which may be used for various applications.